The adenovirus major late transcription unit is a complex transcription unit that encodes upwards of 20 different mRNAs. These mRNAs fall into five families, L1-L5. The members of each family share a poly(A) site, and differ in how they are spliced to the three common leader exons which map near the major late promoter. Expression of these mRNAs is regulated post-transcriptionally. During the early phase of the infection, 3' end processing occurs almost exclusively at the L1 poly(A) site. This, coupled with exclusive use of a single splice acceptor, leads to the production of an L1 mRNA that encodes the 52/55 kDa protein. Late in the infection, all five poly(a) sites are chose at roughly equivalent frequencies, leading to production of the major and minor virion components. In this project, three properties of the major late transcription will be investigated. Second, the parameters allowing equivalent use of all five poly(A) sites during the late phase of the infection will be determined. Third, the reason for expression of the 52/55 kDa protein during the early phase of the infection will be examined. Together, these studies address the how and why of regulation of expression of L1 mRNAs during infection.